In the manufacture of integrated circuits (ICs) and other electronic devices, testing with automatic test equipment (ATE) is performed at one or more stages of the overall process. A special handling apparatus is used to place the device under test (“DUT”) into position for testing. In some cases, the special handling apparatus may also bring the DUT to the proper temperature and/or maintain it at the proper temperature to be tested. The special handling apparatus is of various types including “probers” for testing unpackaged devices on a wafer and “device handlers” for testing packaged parts; herein, “peripheral” or “peripherals” will be used to refer to all types of such apparatus. The electronic testing itself is provided by a large and expensive ATE system. The DUT requires precision, high-speed signals for effective testing; accordingly, the “test electronics” within the ATE, which are used to test the DUT, are typically located in a test head, which must be positioned as close as possible to the DUT. The test head is extremely heavy; the size and weight of test heads have grown over the years from a few hundred pounds to as much as three to four thousand pounds.
In order to use a test head to test integrated circuits, the test head is typically “docked” to a peripheral. When docked, the test head must be located as close as possible to the peripheral's test site in order to minimize signal degradation. A test head positioning system may be used to position the test head with respect to the peripheral and may be designed to facilitate flexible docking and undocking of a test head with a variety of peripherals. A test head positioning system may also be referred to as a test head positioner or test head manipulator. Test head positioning systems have been described in numerous patents.
In the ordinary operation of a test head positioning system, a counterbalanced arm brings and holds the test head into a desired position. Once the test head is in the desired position, the arm may be locked in place. With such systems, if the balance condition is lost while the arm is locked in place and this fact is unknown to the operator of the positioning system, upon release of the locking mechanism moveable parts of the positioning system will move in a rapid and uncontrolled manner.
The risk of imbalance normally comes in the installing, removing or changing of test heads. If the positioner system is locked and then one removes the test head from the arm but forgets to remove the source of the counterbalance (e.g. weights) before unlocking the main arm, then the main arm would fly up. Conversely, if the positioner system is locked and one removes the weights without removing the test head before the lock is released, then the test head falls.
If one adds or removes too many weights prior to unlocking the arm, then there is an imbalance which can cause the test head to fly upwards or downwards after the lock is released.
Finally, while breakage of the cable that couples the arm to the counterbalanced system is unlikely, such breakage is not impossible.
U.S. Pat. No. 4,715,574 provides a safety lock system for a materials handling system such as a test head positioning system. That safety lock system operates in a manner so that if a balance condition is lost while the system is locked, the system cannot be unlocked. More specifically, that system has a safety lock that moves with the main arm. A handle is used for locking the main arm in place. The safety lock prevents rotation of the handle upon a preselected movement of the arm caused by a loss of the balanced condition. If the handle cannot be rotated, then the lock cannot be released.